Monoseeding improves stand establishment through regulation of apical hook formation and hypocotyl elongation in cotton

Cluster seeding is the dominant sowing pattern of cotton in the Yellow River valley of China, but it expends a large amount of seeds and usually results in slender and long-legged seedlings. The subsequent thinning and final singling of seedlings in cluster seeding are also time-and labor-intensive. Monoseeding may be an alternative sowing pattern to solve these problems. In this study, monoseeding, double seeding and cluster seeding treatments were established by sowing one, two and ten seeds each hill to study their effects on emergence, stand establishment and seedcotton yield as well as the underlying physiological and molecular events during emergence. Results showed that monoseeding, double seeding and cluster seeding were not significantly different in seed emergence rate. But in monoseeding pattern, the stand establishment rate and seedling hypocotyl diameter were increased by 16.3-21.2% and 29.2-34.3%, respectively, while the percentage of seedlings with shell attached, disease incidence and hypocotyl length were reduced by 82.2-91.5%, 36.6-37.2% and 26.9-35.8% respectively when compared with those of cluster seedling. These results indicated that monoseeding not only produced stronger seedlings than double or cluster seeding but also improved stand establishment. Additionally, the apical hook angle of seedlings before emergence under monoseeding was 69.1-71.1% smaller than that under cluster seeding, suggesting improved apical hook formation under monoseeding. The ethylene, IAA, and GA contents in seedlings under monoseeding were higher but the JA content was lower than those under cluster seeding during emergence, suggesting that ethylene, IAA, and GA promoted but JA inhibited the apical formation of seedlings. The increased ethylene content under monoseeding was possibly due to the increased expression of ethylene biosynthesis gene ACO1. Monoseeding also increased the expression of apical hook formation promoting genes COP1 and HLS1, but decreased the expression of hook formation inhibiting gene ARF2. The expression of hypocotyl elongation related gene ERF1 under monoseeding increased relative to cluster seeding, which might be an important reason for the shorter and thicker hypocotyl formation under mono seeding than cluster seeding. Monoseeding considerably reduced seeding rate and labor input for thinning and singling of seedlings but produced comparable cotton yield to double or cluster planting. The results suggested that mononseeding improved stand establishment through regulating the expression of apical hook formation and hypocotyl elongation-related genes as well as changes of some endogenous phytohormones. It was also suggested that monoseeding is a promising seeding pattern to reduce seed and labor inputs without sacrificing yield for cotton production in the Yellow River valley of China and other areas with similar ecology.